7.4.1 Operating Principle
The attenuation through an optical fiber varies with frequency and there is particularly low attenuation over specific wavelength bands, notably around: 850 nm (multimode systems), 1310 nm (second window), and latter 1550 nm (third window, lowest attenuation and compatible with optical amplifiers). Modern WDM systems operate primarily in the 1550 nm region because attenuation is minimal (≈0.2 dB km–1 in silica fiber) and because this band is compatible with erbium-doped fiber amplifiers (EDFAs), enabling long-distance transmission without electrical regeneration.
If each optical carrier is modulated at a bit rate Rb, and N wavelengths are multiplexed, the aggregate bit rate becomes:
Thus, WDM increases fiber capacity linearly with the number of wavelengths, without requiring additional fibers. Importantly, WDM increases capacity without increasing the symbol rate of individual channels, in contrast to approaches that raise throughput by transmitting symbols more rapidly.
Conceptually, WDM is the optical analog of FDM. However, unlike electrical FDM—where filters separate frequency bands within a single electrical spectrum—WDM exploits the extremely high carrier frequency of light (on the order of 1014 Hz) which allows extremely fine spectral partitioning compared with electrical systems. This enormous carrier frequency permits many high-bandwidth channels to coexist with negligible overlap relative to the optical carrier.
Key components of a WDM system typically includes:
- Precisely controlled laser sources.
- Optical multiplexers and demultiplexers (implemented using thin-film filters, arrayed waveguide gratings (AWGs), or fiber Bragg gratings).
- Optical amplifiers (e.g., EDFAs).
- Optical add-drop multiplexers (OADMs), allowing selected wavelengths to be inserted or removed without disturbing others.
The ability to selectively add or remove wavelengths forms the basis of reconfigurable optical networks and modern optical transport systems.
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